We have been examining polymorphisms in genes involved in the leptin signaling pathway, to identify gene variants impacting on body composition. We are currently intensively studying a variant MC3R that is associated with adiposity in children and which appears to have functional significance for MC3R signal transduction. Children who were homozygous variant for both the polymorphisms (Thr6Lys and Val81Ile) had significantly greater BMI-SD score, fat mass, body circumference measurements, and higher plasma levels of insulin and leptin compared with wild type or heterozygous children. In vitro studies subsequently found that expression was significantly lower for the double mutant MC3R and suggest degradation of the double mutant MC3R is increased. We have found energy intake is increased but energy expenditures are not altered in children having these polymorphisms. Ongoing studies attempt to understand the mechanisms by which these sequence alterations may impact body weight. We have initiated a new study comparing energy balance during adaptation to a high-fat diet in humans with double mutant and wild type MC3R this year. We have also successfully bred knock-in mice expressing the human wild type and human double-mutant MC3R that were developed in collaboration with Dr. Westphal, and will be studied during the next few years. We have also recently investigated the BDNF-TrkB pathway in regards to body mass in children. Using a comparative genomic hybridization approach, we examined genotype-phenotype relationships in patients with the WAGR (Wilms Tumor, Aniridia, Genitourinary, and Renal abnormalities) syndrome. Compared to those with intact BDNF (BDNF+/+), BDNF+/- had significantly greater body mass during childhood, starting at age 2y. 100% of BDNF+/- were overweight by age 10y vs. only 20% of BDNF+/+ (P<0.0001). A full characterization of the energy intake and expenditure of subjects with WAGR syndrome and other 11p deletion syndromes is underway. We have found that leptin is an important predictor of weight gain in children: those with high leptin gain even more weight when followed longitudinally and identified children with hyperleptinemia and leptin receptor mutations (3). We have also found hyperleptinemia out of proportion with body fat mass in children with psychological loss of control over eating (6). Such data suggest the importance of leptin resistance as a factor stimulating weight gain and have led to recent explorations of other syndromes associated with obesity that may cause dysregulation of leptin signaling, including Bardet Biedl syndrome (in collaboration with Dr. Biesecker) and Alstrom Syndrome. Patients with BBS had higher leptin than expected for their degree of adiposity, consistent with the notion that ciliopathy-induced leptin signaling dysfunction is associated with leptin resistance. We have also recently found evidence for hyperphagia among patients with BBS (10). Other studies are directed at understanding additional genetic, physiological, psychological, and metabolic factors that place children at-risk for undue weight gain (5,8,14,15). We have validated prior associations between body weight and variation in the FTO gene and found SNPs in FTO to be associated with behavioral loss of control over eating. Two recent initiatives target insulin resistance. One trial studies the role of depressive symptoms in childrens insulin resistance. We have found both cross-sectional and prospective longitudinal associations between depressive symptoms and the development of insulin resistance. A randomized clinical trial to study the effects on insulin resistance of preventing depression in obese adolescents with a family history of type 2 diabetes is ongoing. Another pilot study initiated in 2013 examines how sedendary behaviors contribute to poor glucose tolerance. Recent investigations concentrating on binge eating behaviors in children suggest that such behaviors are also associated with adiposity in children, predict future weight gain in children at-risk for overweight, and predict both greater energy consumption during meals and decreased satiety after eating. The ability to consume large quantities of palatable foods (5,8), especially when coupled with decreased subsequent satiety, may play a role in the greater weight gain found in binge eating children. Using our longitudinal cohort, we have also found that the development of some aspects of metabolic syndrome in children is predicted by the presence of binge eating behaviors. These data also suggest that interventions targeting disordered eating behaviors may potentially be useful in preventing excessive fat gain in children prone to obesity. Two ongoing protocols examine efficacy of interpersonal therapy as a weight gain preventive strategy among children and adolescents who report binge eating behaviors. Given the rapid increase in the prevalence of obesity, the development of treatments for obesity is urgently needed. We recently reported efficacy data in Latino children for low-fat and low glycemic load diets, finding both can be successful (7). In other clinical protocols, we have studied pharmacotherapeutic approaches to the control of body weight (11). Two placebo-controlled randomized trials studied the effects of medications on weight loss and on obesity-related comorbidities in children and adolescents. Orlistat 120 mg TID was studied in 200 adolescents, 61% African American, mean BMI 41.70.6 kg/m2. Adolescents treated with orlistat lost more weight (orlistat -2.90.7 vs. placebo -0.60.7 kg, P=0.011), BMI units (-1.720.24 vs. -0.700.24 kg/m2, P=0.002), and fat mass (-3.90.8 vs. -1.40.8 kg, P=0.029), but had little impact on obesity-related co-morbid conditions in overweight adolescents. Metformin 1000 mg BID was studied in 100 severely overweight children (6-12y) who manifested hyperinsulinemia and insulin resistance. Subjects participated in a monthly weight reduction program. Compared to placebo-treated children, those randomized to metformin decreased BMI (metformin -0.910.3 vs. placebo +0.230.3 kg/m2, P=0.006), BMI-Z score (-0.110.02 vs. -0.040.02, P=0.02), and body fat mass (-1.40.7 vs. +2.10.7 kg, P<0.001) to a significantly greater extent. Serum glucose (-2.40.9 vs. +1.61.2 mg/dL, P=0.018), HOMA-IR (-0.190.4 vs. +0.950.4, P=0.05), and total cholesterol (-9.92.7 vs. +1.14.8, P=0.04) also decreased more in metformin-treated compared to placebo-treated children. We concluded that metformin, added to a monthly behavioral program, significantly improved weight loss, insulin resistance, and cholesterol over a 6-month interval in severely overweight, insulin-resistant children. We will report on a 3-year open-label extension protocol to examine the long-term impact of metformin therapy on weight. We expect to initiate additional translational trials in the near future related to modulation of the leptin signaling pathway.